They are often seen lying on tidal reefs and sandbars, where they rest, give birth and suckle their pups in June and July, then molt in August.

Because they are at the top of the food chain, harbor seals serve as an indicator of the health of marine ecosystems.

Aerial survey techniques for land haul-outs, conducted at low tide when peak numbers of animals are hauled out and groups can be photographed, have been fairly standardized for precise counts.

But counting seals on glacial ice has proven more challenging.

In Alaskan waters, hundreds and sometimes thousands of seals congregate at the head of fjords to haul out on icebergs. They are typically dispersed over large areas, often with one or a handful of seals occupying each ice floe. Ice fields are often too large to photograph, or require so many photographs that piecing together a mosaic of the overlapping images with nothing but icebergs as reference points becomes a daunting task.

To develop a better understanding of the behavior and movements of the seals using glacial ice, researchers from National Marine Fisheries Service and Alaska Department of Fish and Game recently launched a collaborative study in Tracy and Endicott Arms, both utilized by up to a thousand seals.

During the last few weeks, researchers have been patiently tending nets set in the ice, waiting for seals to get entangled.

Twenty animals have been fitted with radio transmitters that broadcast signals allowing animals to be tracked and monitored. Fifteen of the animals have also been fitted with time-depth recorders - sophisticated electronic devices that provide a remarkably detailed record of haulout and diving patterns. The devices can record and store data at 10-second intervals on whether the animal was hauled out or in the water, and its depth, over a period of several months.

A series of land-based monitoring stations have also been established near the head and mouth of Tracy and Endicott Arms to keep track of when the animals are utilizing glacial ice and when they leave the fjord.

The study will indicate the fidelity of animals to glacial ice and the extent of movements between the fjords and surrounding areas. Detailed records of haulout patterns will indicate the proportion of animals hauled out and when numbers of seals on ice are at a peak, which will be useful for planning and interpreting surveys. Diving data provided by the time-depth recorders will indicate when during the day or night and depth at which animals are feeding - important clues for determining the prey they might be seeking.

In a companion study, researchers and students have established a small camp at the head of Tracy Arm, and will be observing daily and seasonal changes in abundance and distribution of animals during the pupping season. Various techniques for plotting and tracking seals (and the growing number of tour boats coming to view them) are being tested and evaluated, which will be useful for designing future studies on the impact of vessel disturbances.

In the southern part of their range, harbor seal populations have made a remarkable recovery after having been depleted by bounty kills and commercial harvests for pelts. In British Columbia, for example, numbers increased tenfold during the 1970s and 1980s, and recently stabilized at historic levels. In southeast Alaska, numbers for the most part also appear to be large and either stable or increasing.

In contrast to other regions of Southeast Alaska, seal numbers in Glacier Bay - one of the only glacial sites to be studied in any detail - appear to have declined by half since 1992, according to Beth Mathews, an assistant professor of biology at the University of Alaska Southeast.

However, it is not known whether abundance has actually declined, or whether animals might have moved elsewhere. Little is known to what extent seals on ice might be more vulnerable than those on solid substrates to the disturbances by the growing tour boat and cruise ship traffic, or the wakes they generate (which could rock seals off ice floes over great distances).

Researchers with University of Alaska and National Parks Service have been monitoring seals on glacial ice in Glacier Bay using spotting scopes from cliff-top vantage points and comparing ground counts with high resolution photographs taken with reconnaissance cameras. ADF&G has been evaluating similar camera systems linked with a satellite-based global positioning system that provide precise locations, allowing images to be pieced together over large ice fields. Scientists at National Marine Mammal Laboratory in Seattle are exploring the feasibility of using "national technical means" - jargon for classified technologies that might include such things as the latest generation of reconnaissance satellites.

Scientists hope these studies will shed light on some very basic questions, such as what attracts harbor seals to the ice in the first place. Does the ice field provide a safe refuge from predators such as killer whales? Or do the glacial fjords represent productive feeding areas? Or perhaps the icebergs merely provide a convenient platform for the animals to haul out and rest, pup and molt. Ultimately, the answers to these fundamental questions will provide the basis for understanding and protecting harbor seals and their habitat.

Peter Olesiuk is a research biologist who has been studying marine mammals for about two decades. He recently left the Canadian Department of Fisheries and Oceans and joined the Alaska Department of Fish and Game, where he is principal investigator for the harbor seal program. E-mail Juneau Audubon members at ckent@alaska.net..